Membranes with concentrated solutions

E. Evans and D. Needham Attraction Between Lipid BUayer Membranes in Concentrated Solutions of Nonadsorbing Polymers Comparison of Mean-Field Theory with Measurements of Adhesion Energy. Macromolecules 21, 1822 (1988). 

Ion exchange membranes are not completely semipermeable some leakage of co-ions of the same charge as the membrane can occur. This effect is generally negligible at low feed solution concentrations, but can be serious with concentrated solutions, such as the seawater treated in Japan. 

Preconcentration in a slightly different way is described by Eisner and Mark (40) who equilibrated small areas of cation exchange membranes with sample solutions and then used the membrane as a source of ions for deposition in an anodic stripping voltammetry system. The concentration of the ion in the membrane is linearly related to its concentration in the bulk sample solution. The pre-equilibrated membrane was also analyzed by neutron activation thus extending the range of ions for which the technique is useful. Data are quoted for Ag+, Cu2+, Zn2+, Co2+ and In3+, all of which show favourable distribution for the membrane phase. Equilibration times are inversely proportional to concentration ranging from several minutes at lO"1 M to one day or more at 10 6 M. The method affords a convenient separation from nonionic and anionic species which interfere with the measurement technique. 

The transport number of counter-ions is determined by the difference between the concentration of electrolyte solution in contact with a membrane and the fixed ion concentration of a membrane areas of low fixed ion concentration occur and reduce membrane performance. To obtain a high transport number for counterions in membranes exposed to a higher concentration of solution, the fixed ion concentration of the membrane should be increased to lessen the effect of areas of low fixed ion concentration. Glueckauf et al. determined the heterogeneity of the fixed ion concentration in the membrane by measuring the adsorbed ions in the membrane by equilibrating the membrane with salt solutions of various concentrations.111... 

Wc consider an initial state in which [Figure 6.6(a)] compartment I contains a solution of sodium chloride at a mole fraction xl in contact through a membrane with a solution in compartment II of the sodium salt of a colloidal electrolyte, NaX. The anion X" is unable to diffuse through the membrane. For convenience we assume that the two compartments contain the same amounts of material. Since the concentration of chloride ions in I is greater than that in II, chloride ions will flow from compartment I to II. To maintain electrical neutrality an equal amount of sodium ions must accompany them. Furthermore, diffusion of solvent will also take place to establish osmotic equilibrium. If diffusion lowers the salt concentration in compartment 1 by Ax, the resulting concentrations in the two compartments will have the values shown in Figure 6.6(b). 

The osmotic pressure difference of macromolecular solutions relative to the pure solvent can be represented approximately by zItt = ac", where c is the molar concentration, >1, and a is a constant. This solution is ultrafiltered across a membrane with complete solute rejection. The permeate flux in terms of the applied pressure difference and osmotic pressure difference is assumed to be given by Eq. (4.4.2). 

The two steady states exist within the narrow range of I, i.e. for the same value of I, it can have two values of 4>- In other words, there are two steady states, one with lower resistance and the other with higher resistance. The steady state with lower resistance is obtained when the membrane pores got filled with concentrated solution while the latter corresponds when the membrane pores got filled with dilute solutions. 

Equation (13) describes the net mass flux, Fg/p of solute b relative to stationary coordinates [ 132] at any positiony (0 < f < convective mass flux, Uf t b/f, relative to stationary co-ordinates of solute b toward the membrane with concentration cg/f and solution velocity v, and (2) the diffusive mass flux, F)g (9cb/r/9y), relative to stationary eo-ordinates of solute b away from the membrane along the solute eoneentration gradient (9t b/r/9> )> with diffusion coefficient, )g. All fluxes in this ehapter ean be assumed relative to stationary coordinates [132], so for the sake of brevity, referenee to stationary co-ordinates will be omitted henceforth. The reader will also note that fluxes are vector quantities [132] and the convective and diffusive fluxes at steady state will move solute in opposite directions, hence the opposing signs for the eonveetive and diffusive flux terms relative to stationary co-ordinates in Eq. (13). 

Formamide is teratogenic. The vapor is irritating to the eyes, skin, mucous membranes, and upper respiratory tract. It may be harmful by inhalation, ingestion, or skin absorption. Wear appropriate gloves and safety glasses and always use in a chemical fume hood when working with concentrated solutions of formamide. Keep working solutions covered as much as possible. 

Potentiometric electrodes are divided into two classes metallic electrodes and membrane electrodes. The smaller of these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in solution thus the potential of an Ag wire is determined by the concentration of Ag+ in solution. When another species is present in solution and in equilibrium with the metal ion, then the electrode s potential will respond to the concentration of that ion. Eor example, an Ag wire in contact with a solution of Ck will respond to the concentration of Ck since the relative concentrations of Ag+ and Ck are fixed by the solubility product for AgCl. Such electrodes are called electrodes of the second kind. 

Controlled release can be achieved by a wide range of techniques a simple but important example is illustrated in Eigure 45. In this device, pure dmg is contained in a reservoir surrounded by a membrane. With such a system, the release of dmg is constant as long as a constant concentration of dmg is maintained within the device. Such a constant concentration is maintained if the reservoir contains a saturated solution and sufficient excess of soHd dmg. 

Electrodialysis Reversal. Electro dialysis reversal processes operate on the same principles as ED however, EDR operation reverses system polarity (typically three to four times per hour). This reversal stops the buildup of concentrated solutions on the membrane and thereby reduces the accumulation of inorganic and organic deposition on the membrane surface. EDR systems are similar to ED systems, designed with adequate chamber area to collect both product water and brine. EDR produces water of the same purity as ED. 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved soHds using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaCl solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallisa tion at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

In this lecture we will be concerned by exocytosis of neurotransmitters by chromaffin cells. These cells, located above kidneys, produce the adrenaline burst which induces fast body reactions they are used in neurosciences as standard models for the study of exocytosis by catecholaminergic neurons. Prior to exocytosis, adrenaline is contained at highly concentrated solutions into a polyelectrolyte gel matrix packed into small vesicles present in the cell cytoplasm and brought by the cytoskeleton near the cell outer membrane. Stimulation of the cell by divalent ions induces the fusion of the vesicles membrane with that of the cell and hence the release of the intravesicular content into the outer-cytoplasmic region. 

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